1. Field of Invention
Embodiments of the invention relate generally to methods and apparatus for providing uninterruptible, regulated power to critical and/or sensitive loads. More specifically, embodiments of the invention relate to uninterruptible power supplies (UPS) having improved control systems.
2. Discussion of Related Art
The use of uninterruptible power supplies having back-up systems to provide regulated, uninterrupted power for sensitive and/or critical loads, such as computer systems and other data processing systems, is known. FIG. 1 shows a typical, single phase, on-line UPS 10 used to provide regulated, uninterrupted power. The UPS 10 includes an input circuit breaker/filter 12, a rectifier 14, a control switch 15, a controller 16, a battery 18, an inverter 20 and an isolation transformer 22. The UPS also includes an input 24 for coupling to an AC power source, and an outlet 26 for coupling to a load.
The UPS 10 operates as follows. The circuit breaker/filter 12 receives input AC power from the AC power source through the input, filters the input AC power and provides filtered AC power to the rectifier 14. The rectifier rectifies the input voltage. The control switch 15 receives the rectified power and also receives DC power from the battery 18. The controller 16 determines whether the power available from the rectifier is within predetermined tolerances, and if so, controls the control switch to provide the power from the rectifier to the inverter 20. If the power from the rectifier is not within the predetermined tolerances, which may occur because of “brown out” or “black out” conditions, or due to power surges, then the controller controls the control switch to provide the DC power from the battery to the inverter 20.
The inverter 20 of the UPS 10 receives DC power and converts the DC power to AC power and regulates the AC power to predetermined specifications. The inverter 20 provides the regulated AC power to the isolation transformer 22. The isolation transformer is used to increase or decrease the voltage of the AC power from the inverter and to provide isolation between a load and the UPS. The isolation transformer is typically an optional device, the use of which is typically dependent on UPS output power specifications. Depending on the capacity of the battery and the power requirements of the load, the UPS 10 can provide power to the load during brief power source dropouts or for extended power outages.
Modular UPS systems having redundant control features are also known, and one such system is described in U.S. Pat. No. 5,982,652, the contents of which are incorporated herein by reference. In typical modular UPS systems, additional power modules and battery modules can be added to a UPS system to provide increased output power and longer runtime while on battery.
The use of split-phase power input and outputs in UPS systems is also known. In a typical split-phase UPS, the input of the UPS is configured to receive up to two phases and a neutral from an AC power system and to provide two phases and a neutral at the output of the UPS. Split-phase systems generally provide greater flexibility for a user in that they can support a number of different output power specifications. While split-phase systems generally provide greater flexibility, these systems typically require more complex control systems.
Further, it is often desirable to leave a UPS in a powered down state, and restart the UPS when no AC power is available. Modular systems having this capability are known and include the Symmetra® UPS System available from American Power Conversion Corporation, West Kingston, R.I. In high power, modular systems, it is often difficult to prevent battery drain during a power down mode from which the UPS may be restarted, and it is desirable to minimize the drain that does occur on the batteries.